The present invention relates to airfeed tools, more particularly the present invention relates to air logic circuits for airfeed tools.
The use of peck drilling in heavy industry, particularly industry which requires the drilling of a plurality of holes in a variety of metal pieces, has shown a dramatic increase in recent years. This is because the holes resulting from a peck drilling operation exhibit greater accuracy than holes drilled by conventional drilling operations. Additionally, the surface finish of the hole is much smoother than obtained with conventional drills.
U.S. Pat. No. 4,123,188 to Deremo et al. and U.S. Pat. No. 4,421,441 to Hirose are representative of the design of airfeed peck drills. Such prior art airfeed peck drills include a motor and reduction gearing which is contained within a cylinder to form a drill quill which is attached to the end of an air cylinder. The air cylinder in these prior art drills is used to advance and retract the drill quill into and out of a workpiece. In order to prevent the drill quill from feeding forward before being fully retracted, the back pressure created by the returning quill is used as a signal.
On newer airfeed drills, different sized areas are used on the feed-retract piston. The air logic circuitry as disclosed in U.S. Pat. No. 4,123,188 will not provide the necessary signal to prevent the tool from feeding forward until fully retracted.
Another problem with present Air Feed Peck Drills is the inability to adjust the timing circuitry to compensate for the increased time necessary to actuate the drill bit from the retracted position to the bottom of the hole as the depth of the hole increases. In current air feed peck drills, the timing circuitry which controls the duration of the pecking cycle, or the duration of inward drill stroke, is initiated by the actuation of the peck drill into the working material being drilled, and is not actuated by the actual proximity of the drill bit with the bottom of the hole. As a result, where the drill might rotate through ten revolutions when drilling at the top of the hole, as the hole deepens the time spent at the bottom of the hole, and duration of actual drilling, decreases. In extreme cases, the drill will not reach the bottom of the hole before the timing circuit causes the drill quill to retract.
There is therefore a need in the art to provide an air logic circuit which will compensate for the changing duration of the stroke into the hole as the hole deepens to maintain a relatively constant duration of actual hole drilling.